Reviewing Toni Morrison's last novel, A Mercy (2008), in the New Yorker, John Updike referred to it as "another instalment of her noble and necessary fictional project of exposing the infamies of slavery and the hardships of being African-American". The nobility and necessity of the enterprise does not quite offset the sense of weariness that comes from that "another instalment", and Updike had a point: exposure of infamies and hardship is a fairly limited artistic ambition.

At Morrison's best, in novels such as Beloved (1987) and Song of Solomon (1977), she did much more than expose: she sang, excoriated, harrowed, educated, mythologised and uplifted. It has been 42 years since the publication of The Bluest Eye (1970), her groundbreaking first novel about self-hatred and incestuous rape in the black community. But after nearly half a century, denouncing brutality becomes a fairly circular enterprise. The instalment plan can turn history into a warehouse of horrors: which abuse shall we summon next, which barbarity shall we recount? It is not that novelists should shy away from historical trauma, far from it: but their job is to find something interesting to say about evil, rather than simply announcing its existence, being outraged, and going home. There is no dearth of possibilities, after all: as a species we are deficient in many ways, but we have a talent for atrocity.

Generational legacies, hauntings, ghosts, and the persistent effects of racism and sexism are Morrison's enduring themes: they are big ones. But her novels about them are getting smaller, in every sense; she seems to be losing patience with her own stories. Over the years Morrison's settings have also become increasingly historical, as her novels grow closer to fables: A Mercy went the farthest afield historically, travelling back to the 17th century to tell a revisionist version of the founding of America. Beloved is set in Ohio and Kentucky during the antebellum days of plantation slavery. Many of Morrison's novels range across the 20th century to explore the lingering effects of slavery and poverty, often amid all-black communities: Sula (1973) tells of two women bound by a terrible secret, while Song of Solomon is about the need for people to take flight; Jazz (1992) played with music to tell the story of Harlem during its renaissance in the 1920s. Morrison's last novel set entirely in its contemporary moment was Tar Baby in 1981 (it is also the only one of Morrison's novels not set exclusively in America; much of its action occurs on an imaginary Caribbean island), although the cross-cutting storyline of Love (1993) does reach into the 1990s. Beloved continues to be Morrison's masterpiece, though acknowledgment is rarely made of the strong similarities between Beloved and Corregidora, a remarkable novel by Gayl Jones, which Morrison edited – some say co-wrote – when she worked at Random House in the 70s.

Beloved has been followed by a catalogue of increasingly symbolic abstractions: Paradise (1997), Love (2003), A Mercy and now, Home. Morrison has always had a dangerous tendency toward allegory, and a moralising strain that at its most simplistic can be positively Aesopian. The problem with allegory is that it risks turning literature into a theme park: Take a ride on the Horror Train! Visit the Haunted Mansion of History! (As she moves into history, Morrison is also slipping increasingly into anachronism: A Mercy was riddled with them, and early in Home we are asked to believe that a man in the 1950s thinks that a "face seemed to morph into the front of a Jeep".) In Home, Morrison returns to the 50s, an era she remembers, to mine the traumatic possibilities of the Korean war and of biological experiments on African-Americans. The two themes could have come together neatly – black soldiers were experimented upon, to America's eternal shame – but as one of Morrison's subjects has always been violence against black women, she makes the victim of medical experiments the sister of a soldier.

Home tells the story of Frank Money, an African-American veteran traumatised by his experiences in the Korean war. He has been back in America for a year, but feels too violent and dislocated to go home to Georgia, where his younger sister still lives. As the novel opens, Frank finds himself restrained in a hospital, but can't remember exactly why he's there: "Just the noise. Loud. Real loud … Maybe I was in a fight?" He has received a mysterious letter from a woman named Sarah, telling him that he must hurry home and rescue his younger sister from some unnamed danger: "Come fast. She be dead if you tarry." So Frank breaks out of the hospital, shoeless in the dead of the winter, and begins to make his way cross-country to Georgia, relying on the kindness of strangers and trying to suppress his traumatic memories of the war as he goes. Fortunately for him, the first person he encounters is a kindly minister subtly named John Locke, who gives him $17 and helps him on his way. Morrison cross-cuts Frank's story with that of his sister, Ycidra, known as Cee, who left home at 14 with "a rat" who called himself Prince. He has since run off, and Cee finds a job as a medical assistant for a white doctor named Beauregard Scott; his housekeeper, Sarah, shows Cee his office, where, gazing in awe at titles such as The Passing of the Great Race, and Heredity, Race and Society, she innocently wonders what "eugenics" means. It is a powerful set-up, building suspense and a mounting sense of anxiety: what terrible things are going to happen to Cee, and how will Frank save her, when he can't save himself?

Within this realistic framework, Morrison makes two gestures toward a more experimental sensibility. The first is the insertion of brief, italicised passages in which Frank narrates his own memories and argues with the narrator of the other sections: "Earlier you wrote about how sure I was that the beat-up man on the train to Chicago would turn around when they got home and whip the wife who tried to help him. Not true. I didn't think any such thing. What I thought was that he was proud of her but didn't want to show how proud he was to the other men on the train. I don't think you know much about love. Or me." The other is the mysterious recurrence of a ghostly little man in a pale-blue zoot suit who appears at key moments and then vanishes. Perhaps he is evoking Malcolm X (although Malcolm X was famously tall), who described in his autobiography the sky-blue zoot suit he wore on the streets of Harlem in his hustler days with his "homey" Shorty; Frank also thinks frequently of the "homeys" who went to war with him but did not return home.

This is all very promising, and if Morrison had finished writing the novel she so carefully began, it might have been one of her best in years. But at well under 200 pages with wide margins, Home barely begins before it ends; just when the reader expects the story to kick in to gear, as Frank arrives back in Georgia and finds Cee, Morrison seems to lose interest. Cee's traumatic experience with the doctor is dispatched in a matter of (euphemistic) sentences; Frank simply carries her out of the doctor's house, and they head home to the small town they both hated, where a familiar group of sisterly healing women nurse Cee back to health. Morrison refuses to confront the violence she has invoked, substituting instead a few Morrisonian perorations insisting that a woman own herself ("Don't let … some trifling boyfriend and certainly no devil doctor decide who you are. That's slavery. Somewhere inside you is that free person I'm talking about. Locate her and let her do some good in the world").

Frank's post-traumatic stress disorder disappears as easily, effecting one of the least satisfying "redemptions" I can remember – and like most Americans, I am a sucker for redemption stories. Frank confesses that he is guilty of barbarity during the war – an important confession, especially given the tendency in recent American novels about the Korean war such as Chang-Rae Lee's The Surrendered and Jayne Anne Phillips's Lark and Termite to displace all the cruelty on to secondary characters, keeping protagonists pure and noble – but even as Frank realises he must not "let him[self] off the hook," Morrison does just that. Frank concludes: "The best he could hope for was time to work it loose," and we're done, with the result that it is not only the character who is let off the hook. Home should be relentless, unsparing, but Morrison relents halfway through, and spares everyone – most of all herself.

• This article was corrected on 1 May 2012 because it said that A Mercy was set in the 16th century, rather than the 17th.